1. Field of the Invention
The present invention relates generally to dental blanks for forming dental prostheses. More particularly, the present invention relates to a green body zirconia dental blank with a chemical compositions of increasing amounts of yttria through a thickness thereof and a pre-sintered optical characteristic of non-translucency with respect to visible light that is substantially consistent and opaque across the thickness; and being milled, colored and sintered to form the dental prosthesis with an optical characteristic of increasing translucency through a thickness of the dental prosthesis after sintering.
2. Related Art
There are three main classes of dental ceramics: Group I—predominantly glassy materials; Group II—particle-filled glasses and glass-ceramics as a special subset of particle-filled glasses; and Group III—polycrystalline ceramics.
Group I—predominantly glassy ceramics—are 3-D networks of atoms having no regular pattern to the spacing between nearest or next nearest neighbors, thus their structure is ‘amorphous’ or without form. Glasses in dental ceramics derive principally from a group of mined minerals called feldspar and are based on silica (silicon oxide) and alumina (aluminum oxide), hence feldspathic porcelains belong to a family called alumino-silicate glasses.
Group II—particle-filled glasses and glass-ceramics—have filler particles that are added to the base glass composition in order to improve mechanical properties and to control optical effects such as opalescence, color and opacity. These fillers are usually crystalline but can also be particles of a higher melting glass. Glass-ceramics in Group II have crystalline filler particles added mechanically to the glass, e.g. by simply mixing together crystalline and glass powders prior to firing. In a more recent approach, the filler particles are grown inside the glass object (prosthesis) after the object has been formed. After forming, the glass object is given a special heat treatment, causing the precipitation and growth of crystallites within the glass. Such particle-filled composites are called glass-ceramics. More recently a glass-ceramic containing 70 vol % crystalline lithium disilicate filler has been commercialized for dental use. Example of this is Empress 2, now e.maxPress and e.maxCAD from IvoClar-Vivadent.
Group III—polycrystalline ceramics—have no glassy components; all of the atoms are densely packed into regular arrays that are much more difficult to drive a crack through than atoms in the less dense and irregular network found in glasses. Hence, polycrystalline ceramics are generally much tougher and stronger than group I and II glassy ceramics. Polycrystalline ceramics are more difficult to process into complex shapes (e.g. a prosthesis) than are glassy ceramics and tend to be relatively opaque compared to glassy ceramics. (Ceramic materials in dentistry: historical evolution and current practice (2011), J R Kelly, University of Connecticut Health Center, Department of Reconstructive Sciences, Farminton, Conn.).
Advanced polycrystalline ceramic materials such as zirconia have great potential as substitutes for traditional materials in many biomedical applications. Since the end of the 1990s, the form of partially stabilized zirconia has been promoted as suitable for dental use due to its excellent strength and superior fracture resistance. In addition, zirconia bio-ceramic presents enhanced biocompatibility, low radioactivity, and good aesthetic properties. The introduction of computer-aided design/computer-aided manufacturing (CAD/CAM) techniques has increased the general acceptance of zirconia in dentistry.
Zirconium dioxide (ZrO2) known as zirconia, is a crystalline oxide of zirconium. Although pure zirconium oxide does not occur in nature, it is found in the minerals baddeleyite and zircon (ZrSiO4). At ordinary temperatures, it has a hexagonal close-packed crystalline structure and forms a number of compounds such as zirconate (ZrO3−2) and zirconyl (ZrO+2) salts. Zirconia is obtained as a powder and possesses both acidic and basic properties. Zirconium oxide crystals are arranged in crystalline cells (mesh) which can be categorized in three crystallographic phases: 1) the cubic (C) in the form of a straight prism with square sides 2) the tetragonal (T) in the form of a straight prism with rectangular sides and 3) the monoclinic (M) in the form of a deformed prism with parallelepiped sides. The cubic phase is stable above 2,370° C. and has moderate mechanical properties, the tetragonal phase is stable between 1,170° C. and 2,370° C. and allows a ceramic with improved mechanical properties to be obtained, while the monoclinic phase, which is stable at room temperatures up to 1,170° C., presents reduced mechanical performance and may contribute to a reduction in the cohesion of the ceramic particles and thus of the density.
Partially stabilized zirconia is a mixture of zirconia polymorphs, because insufficient cubic phase-forming oxide (stabilizer) has been added and a cubic plus metastable tetragonal ZrO2 mixture is obtained. A smaller addition of stabilizer to the pure zirconia will bring its structure into a tetragonal phase at a temperature higher than 1,000° C. and a mixture of cubic phase and monoclinic (or tetragonal) phase at a lower temperature. This partially stabilized zirconia is also called tetragonal zirconia polycrystal (TZP). Several different oxides, eg, magnesium oxide (MgO), yttrium oxide, (Y2O3), calcium oxide (CaO), and cerium oxide (Ce2O3), can be added to zirconia to stabilize the tetragonal and/or cubic phases.
Nowadays dental restorations or prostheses are often made using zirconia ceramic with CAD (Computer Aided Design) and CAM (Computer Aided Machining) process, which typically includes:                capturing data representing the shape of a patient's teeth, for example by scanning a plaster model of the patient's teeth or alternatively by scanning the actual teeth in the patient's mouth;        designing the shape of a dental restoration precursor based on the captured data using software, such as computer-aided design (CAD) software;        machining the dental restoration precursor to correspond to the designed shape, for example, by an automated Computer Numerical Controlled (CNC) machine; and        optionally finishing the dental restoration precursor by sintering and/or veneering.        
A common method of making dental restorations includes milling a restoration precursor out of a zirconia disc/blank of a pre-sintered but still porous ceramic material. The disc/blank is typically formed by compacting an amount of ceramic powder. The zirconia disc/blank of compacted powder is usually subsequently pre-sintered to provide it with the required mechanical stability for handling and machining. Once the restoration precursor has been obtained from machining the disc/blank the precursor is typically sintered in the further process of making the final dental restoration. During sintering the precursor typically shrinks, generally proportionally, because the initially porous material reduces in porosity and increases in density. For this reason the restoration precursor may be initially larger, for example about 18 to 27%, than the desired final shape after sintering, to account for shrinkage during the sintering step. To form the final dental restoration the sintered restoration precursor may be veneered or otherwise finished.
Some Group I or II glass ceramic blocks already have different upper and lower optical properties, such as translucency, brightness, reflectance and color. Thus, the glass ceramic block itself already has pre-determined optical properties. For example, see U.S. Pat. No. 8,025,992. Such a pre-colored glass ceramic block can be used primarily in dentists' office with a view to finish the indirect treatment with just one visit. The dental laboratory can also be a user. Here the indirect treatment mainly means to put a crown, bridge, inlay and/or onlay in replacement of the damaged tooth.
After the dentist preps the tooth, he/she chooses a glass ceramic block that already has color in it. Each layer of the block has a color profile already integrated into the block after the pre-sintered stage and it is implied that each layer should not be different in chemical characteristics. For each layer, coloring is affected only by addition of coloring oxides to the melt from which the granulate is obtained, or to the ground granulate and not due to differing chemical characteristics. These oxides are then present separately. In summary, these pre-colored, glass ceramic blocks already have predetermined, built-in optical properties in the block
It can be advantageous for small, ready-to-be-used individual blocks to have these built-in optical characteristics for small production. But pre-colored individual blocks can also be disadvantageous for mass production of prostheses of various sizes and various desired optical characteristics.
The milling machine mills the pre-colored glass ceramic block one at a time in a single mill sequence. The inefficiency with this ceramic block is that if there are 15 different colors of prosthetic teeth to be milled, then the machine should be stopped each time so that the milled block could be removed and each different block could be loaded. Thus, the pre-colored glass ceramic blocks are not efficient for dental laboratories where numerous cases should be milled, regardless of the optical properties of the dental prostheses. These laboratories need to reduce the stopping of the machines as much as possible to save time and increase productivity.
For examples of pre-colored dental blocks, see U.S. Pat. Nos. 8,025,992 and 7,981,531; and US Patent Publication No. 2011-0236855. For examples of zirconia dental blocks, see U.S. Pat. Nos. 7,011,522 and 6,354,836.